Variable intermediate frequency transformer



Aug. 11, 1953 A. EBERHARDT 2,643,825

VARIABLE INTERMEDIATE FREQUENCY TRANSFORMER Filed Nov. 19, 1951 i r t E INVENTOR.

7/00 @[RIVAIRDT A 7- ro uy -r Patented Aug. 11,1953

VARIABLE INTERMEDIATE FREQUENCY TRANSFORMER Arthur Eberhardt, Cedar Rapids, Iowa, assignor to Collins Radio Company, Cedar Rapids, Iowa,

a corporation of Iowa Application November 19, 1951, Serial No. 257,030

4 Claims. 1

This invention relates in general to a constant frequency transformer and in particular to means for maintaining a fixed center frequency while allowing the band width to be changed.

In electronics it is oftentimes desirable to have a variable bandwidth transformer which remains centered at a fixed resonant frequency.

It is an object of this invention therefore to provide an intermediate frequency transformer which has a primary coil mounted on a suitable shaft with a plurality of smaller tertiary coils which may be selectively connected in series with a. secondary coil mounted on a second shaft.

Another object of this invention is to provide a variable intermediate frequency transformer that may be varied in bandwidth while maintaining the center frequency fixed.

Further objects, features and advantages of this invention will become apparent from the following description and claims when read in view of the drawings, in which;

Figure 1 is a side view of the intermediate frequency transformer of this invention;

Figure 2 is a top view of the transformer of this invention;

Figure 3 is the schematic diagram of the transformer; and,

Figure 4 illustrates the response characteristics of the transformer with different tertiary coils connected.

Figure 3 illustrates the equivalent circuit of this transformer. It comprises a primary winding L1, a secondary winding L2, and tertiary windings La, Lb, Lo and Ld.

A condenser C1 is connected in parallel with L1 to form an input tuned circuit. A condenser C2 is connected across the secondary to tune it.

The tertiary windings have one of their ends connected to one end of the secondary L2 and their other ends are connected to terminals engageable with a selector switch Ill. The switch I is connected to the other end of L2.

Figure 1 shows the actual physical embodiment of the transformer wherein the primary L1 is wound on a cylindrical coil form l2 held between end plates I 3 and I4 and which is hollow so that a slug 16 may be moved longitudinally by a lead screw I'I. Connector shafts l8 and I9 hold the end plates together. The tertiary windings La, Lb, Lo and Ld are mounted at varying intervals on the coil form I 2. A second coil form 20 is mounted between end plates I3 and I4 and the secondary winding L2 is mounted thereon. The slug l6 passes through the coils L2 through La at all times. It also passes through L1 but may be moved slightly to tune the input circuit of which L1 forms a part. The condensers C1 and C2 may be mounted to end plates H; or to shafts l8 and I9.

The tertiary coils La through Ld have the same number of turns and thus the inductance of the secondary does not change. The. coils La through La are wound so that some of them oppose the coupling between L1 and L2 and some are wound so as to aid the coupling. The letters s and f in Figure 3. mean start and finish, respectively. Thus, windings La, Lb and Ld aid, and winding Le opposes.

The equivalent coupling between the primary and secondary with the various tertiary coilsmay be given as:

where 1012 is the coefiicient of coupling between windings L1 and L2, kx is the coefficient of cou.-. pling between the particular tertiary coil and L1. L2 is the inductance of L2, and LX- is. the inductance of; the particular tertiary winding. The plus sign corresponds to an aiding coil and the negative sign to an opposing coil. The coupling k12 is constant because L1 and L2 do not move relative to each other. The inductance of coils connected in series is equal to:

(2) Ltotal: (Lm-I-Ly) 2m where m is the mutual inductance between the two coils. In the present invention, the coil L2 is spaced a substantial distance from the tertiary coils so that m becomes negligible and, thus, the inductance does not change in the secondary. If m were a significant term, the Lam would vary when a change were made from aiding to opposing which determines whether the plus or minus sign is used.

Figure 4 illustrates the frequency response of the transformer with various tertiary windings connected. It is to be noted that the characteristic with the contact [0 engaging the winding L2 is broad. Winding Lb gives a narrower bandwidth. This is true because the coupling between the primary coil and Lb is smaller than between the primary and La. Ld is spaced further away from L1 and therefore the coefiicient of coupling is low and a narrow bandwith is obtained. The winding Lo is wound to oppose the secondary winding and gives the characteristic designated as C. The bumps in curves A and B may be smoothed out by inserting series resistors.

It is seen that this invention maintains constant inductance in the secondary while allowing the bandwidth to be varied by choosing tertiary windings that vary in distance from the primary.

Although this invention has been described with respect to the particular embodiment thereof, it is not to be so limited as changes and modifications may be made therein which are within the full intended scope of the invention as defined by the appended claims.

I claim:

l. A variable intermediate frequency transformer comprising, a pair of end plates, a first coil form mounted between the end plates, the primary of said transformer wound about said coil form, a second coil form mounted between said end plates, the secondary of said transformer mounted on said second coil form so as to be loosely coupled to the primary winding, a plurality of tertiary windings all having the same number of turns mounted longitudinally along the first coil form, one side of each tertiary winding connected electrically to one side of the secondary winding, and a selector switch con-, nected to the other side of said secondary winding and selectively engageable with the opposite sides of said tertiary windings.

2. An intermediate frequency transformer comprising, a pair of end plates, a pair of coil forms mounted between said end plates, a primary winding of said transformer mounted on one of said coil forms, a secondary of said transformer wound on the other coil form, a plurality of tertiary coils of equal number of turns mounted on the first coil form and supported at varying longitudinal distances from the primary winding, a longitudinal adjustable slug supported by one of said end plates and extending into the confines of the first coil form, one side of said secondary electrically connected to one side of each of said tertiary coils, and a selector switch connected to the other side of said secondary and selectively engageable with the other sides of said tertiary coils.

3. A variable intermediate frequency transformer comprising, a pair of end plates, a pair of coil forms supported between said end plates, a primary winding wound about one of said coil forms, a plurality of tertiary windings wound about one of said coil forms with each tertiary winding having the same number of turns but some of them wound in opposite directions, a

secondary winding wound about the other form,

a lead screw supported by one of said end plates and extending within the first coil form, a slug attached to the end of said lead screw and passing through the tertiary and primary coils, one side of each tertiary coil connected electrically to one side of the secondary coil, and a selector switch connected to the other side of said secondary coil and selectively connectible to the opposite side of said tertiary coils.

4. An intermediate frequency transformer which remains tuned at a center frequency while allowing the bandwidth to vary comprising, a coil form, a primary winding wound about said coil form, a plurality of tertiary windings wound on the coil form with varying distances from the primary winding and all having the same nume ber of turns, a slug extending through the tertiary and primary windings, a secondary winding loosely coupled to the primary and tertiary windings, one side of the secondary winding connected to one side of each of the tertiary windings, and a selector switch attached to one side of the secondary winding and selectively connectible to the other side of the tertiary windmgs.

ARTHUR EBERHARDT.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 2,364,291 Harvey Dec. 5, 1944 2,407,916 Berg Sept. 17, 1946 FOREIGN PATENTS 1 Number Country Date 795,362 France Jan. 6. 1936 

